I asked my favorite crank grinder today about this myth. He laughed and said no, he has never heard about a crank bending because it was laid flat. He said the crank has a better chance of bending if it fell over and hit the floor.
He also said most cranks he works on are fully counterweighted and would have sufficient support. and if the crank only had counterweights on each end he thinks it could bend. But he has never seen any evidence of this.